Accounting for microorganisms yields stricter water quality criteria and elevated ecological risks of antibiotics: A case study of sulfonamides in the Yangtze River Delta

The derivation of water quality criteria (WQC) for antibiotics is influenced by the inclusion of various organisms' toxicity data, including microbial data, though no definitive conclusions have been reached. This study focuses on sulfonamide antibiotics, common in the Yangtze River Delta (YRD), to assess the influences of different organisms' toxicity data on determining WQCs and subsequent evaluation of ecological risks. A total of 263 toxicity data points from eight sulfonamides, including sulfamethoxazole (SMX) and sulfamethazine (SM2), were selected to derive WQCs using Species Sensitivity Distribution (SSD) methods. Three WQC types were calculated: based on native species (WQC-n), a combination of native and non-native species (WQC-nn), and a combination of species and microorganisms (WQC-nnm). While WQC-nn showed minimal differences from WQC-n, the inclusion of microbial data resulted in more conservative short-term WQCs (SWQC-nnm), calculated as 256.90 μg/L for SMX and 196.09 μg/L for SM2, approximately half of SWQC-nn values (435.20 μg/L for SMX and 491.11 μg/L for SM2). Monitoring data from the past 15 years in the YRD revealed a 133 % increase in ecological risks when using LWQC-nnm compared to LWQC-n and LWQC-nn, particularly under the worst-case pollution scenarios. However, there was a slight temporal decline in overall ecological risks. The study concludes that incorporating microbial toxicity data results in more protective WQCs and underscores the need for further research to develop WQCs that safeguard sensitive organisms and better reflect real-world exposure scenarios, e.g., the mixture exposure.